Lettuce is a popular crop. Worldwide lettuce consumption continues to increase. As a result of this demand, there is a continued need for new lettuce varieties.
Lettuce is an obligate self-pollinating species. This means that the pollen is shed before stigma emergence, assuring 100% self-fertilization. Each lettuce flower is an aggregate of about 10-20 individual florets (typical of the Compositae family).
Hybrid vigor or heterosis occurs in both cross-pollinating and self-pollinating species. Typical presentations of hybrid vigor for lettuce include increased yield, head size and weight, quality, and resistance to unfavorable environmental factors. Another presentation of hybrid vigor is uniformity in maturity, which makes the variety more suitable for mechanical harvesting.
Current methods for crossing of lettuce include methods involving considerable effort. For example, anther tubes may be manually removed from flowers, which though an effective means to ensure the removal of all self-pollinating possibilities, is very tedious and time consuming when a large number of crosses are to be made. Crosses can also be made by misting the designated male flowers to wash the pollen off prior to fertilization. Pollen from another variety or donor parent is then introduced by gently rubbing the stigma and style of the donor parent to the maternal parent. Tags with the pertinent information on date and pedigree are then secured to the flowers in order to keep track. About 3 weeks after pollination, seeds are harvested when the involucre have matured. The seeds are eventually sown and in the presence of markers such as leaf color or leaf margins, the selfed or maternal seedlings or plants are identified. Generally, there are no visible markers and breeders must wait until the F2 generations when expected segregation patterns for the genetic character of interest can be followed. This latter situation mandates a lengthy wait to determine if hybrids are produced. While these methods are effective, they have multiple steps requiring tedious efforts. The structure of the lettuce flower, with a style enclosed by five fused anthers, makes manipulation particularly difficult and reduces the efficiency of these methods.
Another type of crossing method involves the use of male sterility systems. Male sterility has been engineered in lettuce by expression of a ribonuclease gene under the control of a tapetum-specific promoter. Reynaerts et al., Engineered genes for fertility control and their application in hybrid seed production, Scientia Horticulturae (1993) 55 (1-2): 125-129. However such lines are often not completely sterile and have abnormally shaped leaves. Other male sterile systems include the expression of beta-glucanase via a tapetum-specific promoter. Curtis et al., Genomic male sterility in lettuce, a baseline for the production of F1 hybrids, Plant Science Limerick (1996) 113(1): 113-119. Dr. Edward Ryder has described several male sterility systems, but each has characteristics which hinder its use commercially including less predictable segregation ratios, partially fertile “sterile plants,” lack of differentiation between the sterile and fertile plants among others.
Genetically engineered sterility is also available, but is considered GMO and is not a preferred format.
Thus there remains a need in the art for lettuce hybrid production methods not requiring laborious human intervention.